Here's the process to reinstall Firefox 3.6:

You can download an installer for Firefox 3.6 from http://www.mozilla.com/firefox/all-older

Before running it I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information (copy whole profile folder).

Close Firefox 4, and uninstall it. Do not remove your personal data and settings, just the program. Some people report it's okay to skip this step.

Then finally you can reinstall Firefox 3.6. Unless you have installed an incompatible add-on, it should pick up where you left off. If there are serious issues, please post back with details.

Note: I haven't actually tried this myself!

Finally, just a note that System Restore may have changed the registry settings from Firefox 4 to Firefox 3.6, but doesn't actually change the program files, so that could be why it's broken.

I can't back it up and "close Firefox 4" becasue Firefox 4 won't open

NO PROBLEM.

Go to "programs and features" in your control panel. You will see your old version of 3.6 there and NO version 4.

Uninstall version 3.6 DO NOT click the box to delete your stuff.

Go here and re-install version 3.6.17: http://www.mozilla.com/firefox/all-older

This will repair your version 3 and you will be up and running with your old bookmarks, etc.

I've had a dozen calls from people with this problem, and this is the solution.

Enjoy your v.3.6.17